Method of repairing the turbine airfoil nozzle segment

ABSTRACT

A trailing edge coupon for the trailing edge of a airfoil of a turbine nozzle segment is welded to an airfoil section of the nozzle segment having a damaged trailing edge portion removed. The coupon includes film cooling holes spaced one from the other along a pressure side wall portion of the coupon and a plurality of radially spaced openings along the trailing edge. By welding the pressure and suction side edges of the coupon to the pressure and side edges of the remaining portion of the airfoil, the operating life of the nozzle is extended.

The present invention relates to a repair of the trailing edge of anairfoil and particularly relates to a coupon having a trailing edgecooling configuration for replacement of a damaged trailing edge portionof a nozzle airfoil for a gas turbine. The present invention alsorelates to methods for repairing the trailing edge portion of theairfoil.

BACKGROUND OF THE INVENTION

As operational efficiencies in gas turbines have been achieved by higherfiring temperatures and with enhanced aerodynamic design of theairfoils, for example, the nozzle stator airfoils, advanced turbineshave adopted improved cooling configurations particularly for thetrailing edges of the airfoils. It will be appreciated that the hotgases in the gas flow path of turbines may approximate 1700-1800° F. atthe first stage airfoils, e.g., the nozzles. With these higher operatingtemperatures and the advanced aerodynamic designs of the airfoils,improved cooling systems have previously been employed to minimize andslow the rate of damage incurred by the airfoils exposed to these gases.Various configurations of trailing edge cooling systems have previouslybeen proposed and constructed. For example, one such cooling systemincludes a plurality of radially spaced apertures opening through thetrailing edge in combination with radially spaced film cooling holesaxially spaced from and adjacent the trailing edge. The film coolingholes are provided along the pressure side, i.e., the hotter side, ofthe airfoil for film cooling the remaining trailing edge surfaces.Airfoils with such advanced cooling configurations have been provided ingas turbines and have extended the operating life of the turbine nozzlesto nearly twice the operating hours of prior cooling configurations. Asa consequence, the gas turbines may be operated, e.g., by a utility, fora significantly longer period of time before maintenance is requiredwith the attendant cost advantages not only in terms of turbineefficiency but also turbine downtime.

Turbine airfoils have also been previously repaired by replacing damagedtrailing edge portions with a new replacement trailing edge portion.Coupons which constitute a replacement trailing edge portion for anozzle airfoil have previously been welded to the remaining leading andintermediate sections of an airfoil where the damaged trailing edgesections of the airfoil have been removed. Trailing edge coupons,however, have not heretofore been utilized to significantly extend theoperating life of the turbine airfoil as they have lacked the requiredcooling configurations. The nozzles in current use have only a limitednumber of hours of operation before the airfoils of those nozzles are soseverely damaged as to marginalize the efficiency of the turbine andrequire repair. For example, many existing turbines have airfoils with a24,000 hour operation capability. While coupons having similar trailingedge sections as extant in the turbines can be and have been utilized asreplacements for the damaged trailing sections, the hours of operationalcapability of the repaired nozzle airfoils have not been significantlyextended and a similar operating life as the original airfoils have beenanticipated.

Nozzle airfoils are typically provided in a plurality of nozzle segmentsarranged in a circumferential array about the turbine axis. Each segmentincludes inner and outer bands or platforms between which extend one ormore nozzle vanes, i.e., airfoils. The segments are typically cast ofexpensive materials. While it is possible to replace the nozzle segmentsin their entirety with new nozzles, segments having improved coolingconfigurations permitting longer term operation, such replacement nozzlesegments are prohibitively costly particularly in view of the materialsand machining necessary to effect that replacement. Accordingly, thereis a need for a trailing edge coupon and a method of repairing turbineairfoils which will extend the operating life of the repaired nozzlesbeyond the life cycle of the nozzle airfoils originally provided in theturbine.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with a preferred aspect of the present invention there isprovided a trailing edge coupon having enhanced cooling capability foruse as a replacement for a damaged trailing edge portion of a nozzleairfoil enabling an airfoil repair to increase the operating life cycleof the turbine nozzle. The trailing edge coupon includes a plurality offilm cooling holes generally radially spaced from one another along apressure side of the coupon together with a plurality of radially spacedopenings through the trailing edge. A plurality of radially spaced ribsextend between opposite pressure and suction sides of the coupon andforward edges of the coupon are chamfered for welding to remainingportions of nozzle airfoil from which the damaged trailing edge sectionhas been removed. By welding the coupon to the remaining portion of theexisting airfoil, a repaired airfoil having an extended operating lifeis provided. That is, the upgraded airfoil has a cooling configurationcorresponding to newer airfoil configurations with advanced coolingconfigurations enabling extended operating life of the nozzle segments.

In a preferred embodiment of the present invention there is provided anarticle for repairing turbine nozzle segments each having an airfoilbetween inner and outer platforms with a trailing edge portion of theairfoil removed leaving intermediate and leading edge portions of theairfoil between the platforms comprising: a trailing edge coupon havingpressure and suction side wall portions; a plurality of film coolingholes generally radially spaced one from the other along the pressureside wall portion of the coupon, the coupon including a trailing edgehaving a plurality of radially spaced openings and a plurality ofradially spaced ribs extending between opposite pressure and suctionsides of the coupon, edges of the pressure and suction side wallportions being chamfered for welding to pressure and suction side wallportions of the intermediate portion of the airfoil.

In a further preferred embodiment of the present invention, there isprovided a repaired turbine nozzle comprising: leading edge andintermediate airfoil sections and inner and outer platforms formingremaining portions of a nozzle segment having a removed damaged trailingedge section; a trailing edge coupon having a trailing edge, pressureand suction side wall portions, a plurality of film cooling holesgenerally radially spaced one from the other along a pressure side wallportion of the coupon, a plurality of radially spaced openings along thetrailing edge, and a plurality of radially spaced ribs extending betweenopposite pressure and suction sides of the airfoil, welds along oppositepressure and suction side edges of the remaining section and respectivepressure and suction side edges of the trailing edge coupon whereby theremaining section and the coupon form a complete airfoil between theinner and outer platforms.

In a still further preferred embodiment of the present invention thereis provided a method of repairing an airfoil of a turbine nozzle segmentcomprising the steps of: (a) removing a damaged trailing edge portion ofthe airfoil leaving a remaining airfoil section including leading edgeand intermediate sections; (b) providing a trailing edge coupon havingpressure and suction side wall portions, a plurality of film coolingholes generally radially spaced one from the other along a pressure sidewall portion of the coupon and a plurality of radially spaced openingsand a plurality of radially spaced ribs extending between oppositepressure and suction sides of the airfoil; and (c) welding alongopposite pressure and suction sides of each the remaining airfoilsection and the coupon respectively to secure the coupon to theremaining airfoil section.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary axial view of a plurality of nozzle segmentsforming part of an annular array of nozzles for a turbine stage;

FIG. 2 is a side elevational view of a nozzle segment installed betweenportions of an inner and outer casing;

FIG. 3 is an enlarged cross-sectional view through the airfoils of anozzle segment illustrating a coupon welded to a reusable portion of theoriginal airfoil;

FIG. 4 is an enlarged cross-sectional view of the coupon for repairingthe nozzle airfoil;

FIG. 5 is an enlarged side elevational view of the coupon of FIG. 4;

FIG. 6 is an enlarged view of the film cooling holes along the pressureside of the coupon; and

FIGS. 7-9 are schematic illustrations of the manner of repairing thenozzle airfoil.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, there is illustrated a plurality ofcircumferentially spaced nozzle segments, generally designated 10,forming part of a gas turbine and arranged in an annular array about aturbine axis. It will be appreciated that each of the segments 10includes one or more nozzle vanes or airfoils 12 which extend between aninner platform 14 and outer platform 16. The nozzle airfoils 12 as wellas the platforms 14 and 16 define in part a generally axially extendinghot gas path through the turbine as well as defining a portion of aturbine stage. In FIGS. 2 and 3, the nozzle segments 10 are illustratedin greater particularity. In this embodiment, a pair of airfoils 12 formpart of each segment 10 and extend between the inner and outer platforms14 and 16, respectively. It will be appreciated that each segment mayinclude one or more airfoils between the platforms. Also illustrated inFIG. 2 is an retaining ring 20 to which the nozzle segment is securedand an inner support ring 22 having a slot 24 for receiving a radiallyinwardly extending rail 26 of the inner platform 14 for securing thenozzle segment against axial movement in an aft direction.

As noted previously, the nozzle airfoils 12 frequently become crackedand oxidized in use and must be replaced. It will be appreciated that toreplace the nozzle segments in their entirety is not cost effective inview of the cost of the segments and involved labor, as well as thedowntime required for replacement. Moreover, replacement of worn nozzleairfoils with new airfoils typically leads to similar cracking andoxidation over time necessitating replacement once again over asimilarly limited operating cycle. Consequently, there is a need for aneffective repair which will not only reduce repair costs but will alsoextend the life of the nozzle airfoil to a greater extent that theoriginal airfoil.

To accomplish the foregoing, it will be appreciated that the portion ofthe nozzle airfoil subjected to the majority of the cracking andoxidation is the trailing edge region. Regions of the nozzle airfoilforward of the trailing edge region are not subjected to the samemagnitude of cracking and oxidation and therefore have extended life ascompared with the trailing edge region. As a consequence, it is possibleto remove the damaged trailing edge portion of the nozzle airfoil andreplace that portion with a trailing edge portion or coupon which hasimproved cooling characteristics enabling the life of the repairednozzle airfoil to be extended significantly beyond the life of theoriginal airfoil, e.g., twice the life of the original airfoil.

Referring to FIGS. 4-6, a trailing edge coupon 30 is provided havingpressure and suction side wall portions 32 and 34 respectivelyterminating in a trailing edge 36. Coupon 30 constitutes a replacementfor a removed damaged trailing edge portion of the original airfoil. Thecoupon 30 is provided with a cooling configuration which slows the rateof oxidation of the trailing edge of the repaired airfoil and hence thecoupon therefore enables the repaired airfoil to have an extendedoperating life cycle. While the cooling configuration of the coupon 30is similar to the cooling configuration of original airfoils currentlyin use, those cooling configurations have not heretofore been utilizedin a coupon for repair of damaged nozzle airfoils.

Referring particularly to FIGS. 4-6, the coupon 30 includes a pluralityof ribs 38 which extend in the trailing edge region 40 at generallyradially spaced positions between the pressure and suction sides 32 and34 respectively. The radial spacing between the ribs 38 forms radiallyspaced flow channels for directing cooling air from the interior of therepaired airfoil in a generally axially aft direction for reception in aplenum 42. Plenum 42 extends substantially radially between the innerand outer extremities of the nozzle airfoil. A plurality of radiallyspaced passages 44 are in communication with the plenum 42 and extend ina generally axial direction terminating in exit apertures in thetrailing edge 36 of the coupon 30. Thus, it will be appreciated thatcooling air flowing within the repaired airfoil flows generally in anaxial direction through the channels between the ribs 38 forconvectively cooling the sides of the airfoil and exits into the plenum42. The cooling air is then discharged through the passages 44 and thetrailing edge 36 cooling the opposite sides of the airfoil as the airtraverses from plenum 42 to the trailing edge 36.

Additionally, along the pressure side 32 of the coupon 30, there areprovided a plurality of generally spaced film cooling holes 46. Coolingair within the repaired airfoil also flows through the film coolingholes 46 and along the pressure side wall 32 of the repaired airfoil tofilm cool the pressure side trailing edge region. The film cooling holes46 have external surfaces along the pressure side which are flared asindicated at 48 in FIG. 6. The flare enlarges in the radial and axiallyaft directions such that the film of cooling air is distributed tooverlie substantially the entirety of the pressure side surface of thetrailing edge region 40 to the trailing edge 36. With the film coolingholes 46, the flared surfaces thereof as well as the cooling passages 44exiting through the trailing edge 36, the repaired airfoil containingsuch cooling configuration has reduced oxidation and potential forcracking.

To repair the damaged airfoil and with reference to FIGS. 7-9, thenozzle segments are removed from the turbine. The damaged trailing edgeregions 50 of the airfoils of the removed segments are removed from theairfoils 12. Particularly, the airfoils are cut along both the pressureand suction sides a predetermined distance from the trailing edge andare also cut along the juncture of the trailing edge region beingremoved and the inner and outer platforms 14 and 16, respectively. Thecutting may be effected by a water jet cutter, wire EDM or a plasmatorch as schematically illustrated at 52 in FIG. 7. The trailing edgeregion 50 which has been damaged is thus removed as illustrated in FIG.8 leaving discrete edges along the pressure and suction sides of theoriginal airfoil. Those edges are cleaned and shaped to receivecorresponding edges 54 and 56 (FIG. 4) of the coupon 30. The remainingportions of the airfoil include leading edge of intermediate portions 53and 55, respectively.

Particularly, as illustrated in FIG. 9, the coupon 30 is disposedbetween the inner and outer platforms and the pressure and suction sideedges 54 and 56 are welded to the pressure and suction side edges of theremaining portion of the airfoil to form a repaired airfoil having thesame aerodynamic configuration as the original airfoil. Additionally,however, it will be appreciated that the cooling configuration of thetrailing edge of the repaired airfoil has been changed by the removal ofthe damaged trailing edge region 50 and the addition of the coupon 30with the aforedescribed cooling configuration.

The weld-up of the coupon 30 to the remaining portion of the airfoil maybe accomplished for example by a TIG welding process. The welds 57proceed along the opposite sides of the airfoil between the inner andouter platforms and also between the radial outer and inner edges of thecoupon and the outer and inner platforms. The welds are cleaned toprovide a smooth surface and fillets 59 (FIG. 9) are applied asnecessary and blended with the repaired airfoils having the advancedcooling configuration in each of these segments.

The segments are then replaced into the turbine. Because of the newcooling configuration provided by the coupon repair, the operating lifecycle of the nozzle airfoils is significantly increased. Cracking andoxidation are reduced and the interval between repairs is greatlyincreased, e.g., by a factor of two.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1-6. (canceled)
 7. A method of repairing an airfoil of a turbine nozzlesegment comprising the steps of: (a) removing a damaged trailing edgeportion of the airfoil leaving a remaining airfoil section includingleading edge and intermediate sections; (b) providing a trailing edgecoupon having pressure and suction side wall portions, a plurality offilm cooling holes generally radially spaced one from the other along apressure side wall portion of the coupon and a plurality of radiallyspaced openings, and a plurality of radially spaced ribs extendingbetween opposite pressure and suction sides of the airfoil; and (c)welding along opposite pressure and suction sides of each the remainingairfoil section and the coupon respectively to secure the coupon to theremaining airfoil section.
 8. A method according to claim 6 includingwelding opposite radial edges of the coupon to inner and outerplatforms.
 9. A method according to claim 6 including forming a filletat the juncture of the opposite radial edges of the coupon and the innerand outer platforms respectively.
 10. A method according to claim 6wherein step (b) includes providing a trailing edge coupon having thenumber of the openings exceeding the number of the holes by a factor ofat least two.